Low power circuit structure with metal gate and high-k dielectric

ABSTRACT

FET device structures are disclosed with the PFET and NFET devices having high-k dielectric gate insulators, metal containing gates, and threshold adjusting cap layers. The NFET gate stack and the PFET gate stack each has a portion which is identical in the NFET device and in the PFET device. This identical portion contains at least a gate metal layer and a cap layer. Due to the identical portion, device fabrication is simplified, requiring a reduced number of masks. Furthermore, as a consequence of using a single layer of metal for the gates of both type of devices, the terminal electrodes of NFETs and PFETs can be butted with each other in direct physical contact. Device thresholds are further adjusted by oxygen exposure of the high-k dielectric. Threshold values are aimed for low power consumption device operation.

FIELD OF THE INVENTION

The present invention relates to power conserving electronic circuits.In particular, it relates to circuit structures having high-k containinggate dielectrics and metal containing gates. The invention also relatesto ways of adjusting the threshold voltages for suiting low poweroperation.

BACKGROUND OF THE INVENTION

Today's integrated circuits include a vast number of devices. Smallerdevices and shrinking ground rules are the key to enhance performanceand to reduce cost. As FET (Field-Effect-Transistor) devices are beingscaled down, the technology becomes more complex, and changes in devicestructures and new fabrication methods are needed to maintain theexpected performance enhancement from one generation of devices to thenext. The mainstay material of microelectronics is silicon (Si), or morebroadly, Si based materials. Among others, one such Si based material ofsignificance for microelectronics is the silicon-germanium (SiGe) alloy.The devices in the embodiments of the present disclosure are typicallypart of the art of single crystal, Si based material device technology.

There is a great difficulty in maintaining performance improvements indevices of deeply submicron generations. Therefore, methods forimproving performance without scaling down have become of interest.There is a promising avenue toward higher gate dielectric capacitancewithout having to make the gate dielectric actually thinner. Thisapproach involves the use of so called high-k materials. The dielectricconstant of such materials is significantly higher than that of SiO₂,which is about 3.9. A high-k material may physically be significantlythicker than oxide, and still have a lower equivalent oxide thickness(EOT) value. The EOT, a concept known in the art, refers to thethickness of such an SiO₂ layer which has the same capacitance per unitarea as the insulator layer in question. In today state of the art FETdevices, one is aiming at an EOT of below 2 nm, and preferably below 1nm.

Device performance is also enhanced by the use of metal gates. Thedepletion region in the poly-Si next to the gate insulator can become anobstacle in the path to increase gate-to-channel capacitance. Thesolution is to use a metal gate. Metal gates also assure goodconductivity along the width direction of the devices, reducing thedanger of possible RC delays in the gate.

Low power consumption small FET devices are in need of precise thresholdvoltage control. As operating voltage decreases, to below 2V, thresholdvoltages also have to decrease, and threshold variation becomes lesstolerable. Every new element, such as a different gate dielectric, or adifferent gate material, influences the threshold voltage. Sometimessuch influences are detrimental for achieving the desired thresholdvoltage values. Any technique which can affect the threshold voltage,without other effects on the devices is a useful one. Such techniquesexist, for instance by using so called cap layers, or by exposing high-kgate dielectrics to oxygen.

Unfortunately, shifting the threshold of both PFET and NFET devicessimultaneously, may not easily lead to threshold values in an acceptablytight range for CMOS circuits. There is need for a structure and atechnique in which the threshold of one type of device can beindependently adjusted without altering the threshold of the other typeof device.

Typically, small FET devices with high-k dielectrics and metal gatesrequire expensive complicated processing. It would be useful to findways to simplify the fabrication process, while maintaining most of theperformance offered by such advanced structures. At the same time itwould also be desirable to adjust thresholds for low power operation. Todate, such a structure, or fabrication process does not exists.

SUMMARY OF THE INVENTION

In view of the discussed difficulties, embodiments of the presentinvention disclose a circuit structure which includes at least one NFETand at least one PFET device. The NFET includes an n-channel hosted in asingle crystal Si based material, and an NFET gate stack overlapping then-channel. The PFET includes a p-channel hosted in a single crystal Sibased material, and a PFET gate stack overlapping the p-channel. TheNFET gate stack and the PFET gate stack each has a portion which isidentical in the NFET device and in the PFET device. This portionincludes at least a gate metal layer and a cap layer, where the gatemetal layer and the cap layer are directly interfacing with each other.The NFET device further includes an NFET gate insulator, where the NFETgate insulator further includes a layer of a first high-k material,where the layer of the first high-k material is directly interfacingwith the cap layer of the NFET device. The PFET device further includesa PFET gate insulator, where the PFET gate insulator further includes alayer of a second high-k material, where the layer of the second high-kmaterial is directly interfacing with the cap layer of the PFET device.The absolute values of the saturation thresholds of both the NFET andthe PFET devices are higher than about 0.4 V.

Embodiments of the present invention further disclose a method forproducing a circuit structure. The method includes: in the fabricationof an NFET, implementing an NFET gate stack, an NFET gate insulator, andan n-channel, where the n-channel is hosted in a Si based material andunderlies the NFET gate insulator, where the NFET gate insulatorincludes a layer of a first high-k material. The method furtherincludes: implementing a PFET gate stack, a PFET gate insulator, and ap-channel, where the p-channel is hosted in the Si based material andunderlies the PFET gate insulator, where the PFET gate insulatorincludes a layer of a second high-k material. One overlays the firsthigh-k material and the second high-k material with a layer of a capmaterial, where the first and second high-k materials are in directphysical contact with the layer of the cap material, and overlays thecap material with a layer of a gate metal, where the layer of the capmaterial and the layer of the gate metal are in direct physical contact.During implementation of the NFET gate stack and the PFET gate stack,producing a portion in the NFET gate stack and in the PFET gate stack bypatterning the layers of the cap material and of the gate metal. Thisportion is identical in the NFET device and in the PFET device. Themethod also teaches the overlaying the NFET gate stack and a vicinity ofthe NFET gate stack with a first dielectric layer, and exposing the NFETdevice and the PFET device to oxygen. The oxygen reaches the secondhigh-k material, and causes a predetermined shift in the thresholdvoltage of the PFET device, while due to the first dielectric layeroxygen is prevented from reaching the first high-k material and the NFETdevice threshold remains unchanged.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the present invention will become apparentfrom the accompanying detailed description and drawings, wherein:

FIG. 1 shows a schematic cross section of a circuit structure accordingto an embodiment of the present invention, including identical gatemetal layer and cap layer in both type of devices, and compressive andtensile dielectric layers;

FIG. 2 shows a schematic cross section of a stage in the processingwhere various layers, including layers common in both type of devices,have been deposited;

FIG. 3 shows a schematic cross section of a stage in the processingafter gate patterning;

FIG. 4 shows a schematic cross section of a state of the processing forembodiments of the present invention where gate stacks and electrodeshave already been formed;

FIG. 5 shows a schematic cross section of a following stage in theprocessing of embodiments of the present invention where spacers havebeen removed;

FIG. 6 shows a schematic cross section of a stage in the processing ofembodiments of the present invention where an oxygen blocking stresseddielectric layer is overlaying the NFET device, and the circuitstructure is exposed to oxygen; and

FIG. 7 shows a symbolic view of a processor containing at least onecircuit structure according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

It is understood that Field Effect Transistor-s (FET) are well known inthe electronic arts. Standard components of a FET are the source, thedrain, the body in-between the source and the drain, and the gate. Thebody is usually part of a substrate, and it is often called substrate.The gate is overlaying the body and is capable to induce a conductingchannel in the body between the source and the drain. In the usualnomenclature, the channel is hosted by the body. The gate is separatedfrom the body by the gate insulator. There are two type of FET devices:a hole conduction type, called PFET, and an electron conduction type,called NFET. Often, but not exclusively, PFET and NFET devices are wiredinto CMOS circuits. A CMOS circuit contains at least one PFET and atleast one NFET device. In manufacturing, or processing, when NFET andPFET devices are fabricated together on the same chip, one is dealingwith CMOS processing and the fabrication of CMOS structures.

In FET operation an inherent electrical attribute is the thresholdvoltage. When the voltage between the source and the gate exceeds thethreshold voltage, the FETs are capable to carry current between thesource and the drain. Since the threshold is a voltage differencebetween the source and the gate of the device, in general NFET thresholdvoltages are positive values, and PFET threshold voltages are negativevalues. Typically, two threshold voltages are considered in theelectronic art: the low voltage threshold, and the saturation threshold.The saturation threshold, which is the threshold voltage when a highvoltage is applied between the source and the drain, is lower than thelow voltage threshold. Usually, at any given point in the technology'sminiaturization, devices in lower powered circuits have higherthresholds and typically lower performance than the device thresholds ofless power constrained higher performance circuits.

As FET devices are scaled to smaller size, the traditional way ofsetting threshold voltage, namely by adjusting body and channel doping,loses effectiveness. The effective workfunction of the gate material,and the gate insulator properties are becoming important factors indetermining the thresholds of small FETs. Such so called small FETs havetypically gate, or gate stack, lengths less than 60 nm, and operate inthe range of less than about 1.7 V. The gate stack, or gate, length isdefined in the direction of the device current flow, between the sourceand the drain. For small FETs the technology is progressing toward theuse of metallic gates and high-k dielectric for gate insulators.

In small devices with low EOT gate insulators, the workfunction of thegate may significantly influence the threshold voltage. In the generalterminology of the art, one characterizes the workfunction of the gatein relation to the Si energy gap. For instance, in the art the term“band-edge workfunction” means that the gate has a workfunction likethat of n⁺, or p⁺ Si. Similarly, “mid-gap”, or “quarter-gap”workfunction mean a gate appearing to have a workfunction roughly likeintrinsic silicon, or one halfway between intrinsic Si and heavily dopedSi. All other things being equal, with only the gate workfunctionchanging, the threshold difference for a small device is in the range ofabout 0.3 V-0.5 V for a gate workfunction going from band-edge value tomid-gap value.

The gate insulator also may influence the device threshold. Any chargepresent in the gate insulator, or on the interfaces of the gateinsulator, does change the device threshold. Various high-k materialsused in gate insulators of small devices do have effects on the devicethreshold. It is know that exposing a gate dielectric which comprises ahigh-k material to oxygen, may result in shifting device thresholds in adirection which is the same as if one moved the gate workfunction towardthe p⁺ silicon workfunction. This results in lowering the PFET devicethreshold, namely, making it a smaller negative voltage, and raising theNFET device threshold, namely making it a larger positive voltage. Fromthe way the device thresholds behave, oxygen exposure of the high-kmaterial results in rendering the high-k material more negative relativeto its state before the oxygen exposure. This may mean actualaccumulation of a net negative charge, or possibly the decreasing of analready present positive charge. In either case, the net chargeconcentration of the high-k material, which would include possiblecharges on the interfaces of the material, shifts toward the negativedirection due to the oxygen exposure. Such threshold shifts due tooxygen diffusion to high-k gate dielectrics have been reported in theart, for instance: “2005 Symposium on VLSI Technology Digest ofTechnical Papers”, Pg. 230, by E. Cartier.

It is preferable to carry out such an oxygen exposure at relatively lowtemperatures, and it is also preferable that no high temperatureprocessing should occur afterwards. Accordingly, such a thresholdshifting operation should occur late in the device fabrication,typically after the source and the drain have been activated. Thisrequirement means that one has to expose the high-k material in the gatedielectric at a point in the fabrication process when substantially mostof the processing has already been carried out, for instance, the gate,and gate sidewalls are all in place, and the gate insulator is shieldedunder possibly several layers of various materials. However, there maybe a path for the oxygen to reach from the environs to the gateinsulator. This path is through oxide, SiO₂, based materials, ordirectly and laterally through the high-k material itself. Oxidetypically is the material of liners. Liners are thin insulating layerswhich are deposited conformally essentially over all of the structures,in particular over the gates and the source/drain regions. Use of linersis standard practice in CMOS processing. From the point of view ofadjusting the threshold of the device, the property of interest is thatthe liner would be penetrable by oxygen. Indeed, as referenced earlier,such threshold shifts due to oxygen diffusion through liners, are knownin the art. Additional layers that may separate a gate insulator fromthe environment after the source and the drain have already beenfabricated, are so called offset spacers. As known in the art, offsetspacers are usually on the side of the gate, fulfilling the same rolefor source/drain extension and halo implants, as the regular spacersfulfill in respect to the deeper portions of the source/drain junctions.Offset spacers may typically also be fabricated from oxide.Consequently, if a FET is exposed to oxygen, when a liner and an offsetspacer are covering the gate, the oxygen may reach the gate insulatorwithin a short time, measured in minutes our hours. However, in anygiven particular embodiment of FET fabrication there may be furtherlayers, or fewer layers, covering the gate after the source/drainfabrication, but as long as they do not block oxygen, they are notforming an obstacle to adjusting the threshold by oxygen exposure.

It would be preferable if the thresholds of the different types ofdevices could be adjusted individually, meaning, one would desire to usethreshold tuning techniques, such as the oxygen exposure, in a mannerthat the threshold of one type device becomes shifted without affectingthe threshold of the other type of device. Embodiments of the presentinvention teach such a selective adjusting of a device threshold byhaving oxygen diffusing to the gate dielectric of one type of FET, whilethe other type of FET is not affected. The device not to be affected bythe oxygen exposure is covered by a dielectric layer which does notpermit oxygen penetration. Such an oxygen blocking dielectric layer maybe of nitride (SiN). In embodiments of the present invention the nitridelayer is not only used to block oxygen, but it is deposited in suchconditions that it is in a stressed state, and it imparts this stressedstate onto the channel of the FET. This stress in the channel results inhigher device performance. After the oxygen exposure, the device withthe changed threshold also receives an appropriately stressed dielectriclayer mainly in order to improve its performance.

Some metals, or metallic materials suitable for gate metals, forinstance W, under standard deposition circumstances behave as mid-gapworkfunction materials. It was studied and observed that sandwiching aso called cap layer between the high-k material of the gate insulatorand the gate metal, and with proper treatment, the effectiveworkfunction of the gate metal can be shifted toward the n⁺ Si band-edgevalue. The effect of such cap layers has been already reported in theart, see for instance, by V. Narayanan et al in “IEEE VLSI Symposium”,p. 224, (2006), and by Guha et al. in Appl. Phys. Lett. 90, 092902(2007).

The behavior of the metal workfunction, and that of the high-k materialallows one to fabricate a complex circuit structure, containing bothNFET and PFET devices, with an extremely simple process, which processmay yield low power, but high density, and relatively high performancecircuits. Simplicity in a process means cost savings, both because ofthe less cumbersome process, and due to a presumably higher yield. Lowpower, is an important characteristic, since the art is approaching thelimits of the system cooling capacity due to circuit power consumption.

Embodiments of the present invention achieve simplicity by fabricatingthe gate metal layers and the cap layers for both type of devices fromuniformly deposited common layers. In this manner large number ofmasking and processing steps are saved, in comparison with the usualprocedures in the art where he fabrication of the two type of devices isnot compatible, and where while processing one of the device types theother type has to be masked. Furthermore, in representative embodimentsof the present invention the gate insulator, including the high-kmaterial, is also commonly processed for both type of devices usingblanket layers of the gate dielectrics.

FIG. 1 shows a schematic cross section of a circuit structure 100according to an embodiment of the present invention, including identicalgate metal layer and cap layer in both type of devices, and compressiveand tensile dielectric layers. The figure depicts two devices, an NFETand a PFET, of the at least one NFET and PFET device that make up thecircuit structure, typically a CMOS structure.

It is understood that in addition to the elements of the embodiments ofthe invention, the figures show several other elements since they arestandard components of FET devices. The device bodies 50 are of a Sibased material, typically of a single crystal. In a representativeembodiment of the invention the Si based material bodies 50 areessentially Si. In exemplary embodiments of the invention the devicebodies 50 are part of a substrate. The substrate may be any type knownin the electronic art, such as bulk, or semiconductor on insulator(SOI), fully depleted, or partially depleted, FIN type, or any otherkind. The bodies 50 are hosting, respectively, an n-channel 44 and ap-channel 46 for the two type of devices. Substrates, or bodies 50, mayhave various wells of various conductivity types, in various nestedpositioning enclosing device bodies. These, as many other nuances arenot displayed, or discussed further, as having no particularsignificance for embodiments of the present disclosure. The figure showswhat typically may be only a small fraction of an electronic chip, forinstance a processor, as indicated by the wavy dashed line boundaries.Typically the devices have silicide 42 at the top of the gate stacks 55,56. As one skilled in the art would know, these elements all have theirindividual characteristics. Accordingly, when common indicators numbersare used in the figures of the present disclosure, it is because fromthe point of view of embodiments of the present invention the individualcharacteristics of such elements having no particular significance.

The devices have standard sidewall offset spacers 30, 31. Forembodiments of the present invention the offset spacer material is ofsignificance only to the extent that the offset spacer 31 pertaining tothe PFET device, the one which had its threshold adjusted by oxygenexposure, is preferably penetrable to oxygen. The typical material usedin the art for such spacers is oxide, satisfying the oxygenpenetrability requirement. Typically the spacer of the NFET device 30and the spacer of the PFET device 31 are fabricated during the sameprocessing steps, and are of the same material. However, forrepresentative embodiments of the present invention the offset spacers30, 31 are not essential, and may not be employed at all, or may beremoved before the structures are finalized.

The devices also show liners 21, 22 as known in the art. Such liners areregularly used in standard CMOS processing. The material of such linersis an oxide, typically silicon-dioxide (SiO₂). The traditional role forthe liners is in the protection of the gate stacks 55, 56 and thesource/drain structure regions during various processing steps,particularly during etching steps. Such liners typically have selectiveetching properties relative to nitride layers and silicon. The materialof the PFET liner 21, typically SiO₂, allows oxygen diffusion, affordingoxygen to reach the gate dielectric 11. When oxygen reaches the gateinsulator 11, it can shift the threshold voltage of the PFET by adesired, predetermined amount.

Both the NFET gate stack and the PFET gate stack has a portion 110 whichis identical in the NFET device and in the PFET device. This portion 110contains in each device at least a gate metal layer 70 in the NFETdevice and 71 in the PFET device, and a cap layer 72 in the NFET deviceand 73 in the PFET device. In the NFET device the gate metal layer 70and the cap layer 72 are directly interfacing, and in the same manner,in the PFET device the gate metal layer 71 and the cap layer 73 are alsodirectly interfacing.

Portion 110 in both gate stacks 55, 56 being identical, entails that thematerial in gate metal layers 70, 71 is the same, and separately, thatthe material in the cap layers 72, 73 is the same, as well. Inembodiments of the present invention the material of the gate metallayers may be selected from the group consisting of W, Mo, Mn, Ta, TaN,TiN, WN, Ru, Cr, Ta, Nb, V, Mn, Re, and their admixtures. For reasons ofsetting thresholds to the desired range, typically those metals that canbe deposited with a roughly mid-gap to quarter-gap, workfunction, suchas W and TiN, are selected in representative embodiments. Typically thegate metal layers 70, 71 in both devices are composed essentially ofTiN. In embodiments of the present invention the material of the caplayers 72, 73 may contain materials form Group IIA and/or Group IIIB ofthe periodic table. In representative embodiments of the invention thecap layers 72, 73 may contain lanthanum (La).

The NFET device further has an NFET gate insulator. The NFET gateinsulator includes a layer of a first high-k material 10. This layer offirst high-k material 10 directly interfaces with the cap layer 72 inthe NFET device. The PFET device further has a PFET gate insulator. ThePFET gate insulator includes a layer of a second high-k material 11.This layer of second high-k material 11 directly interfaces with the caplayer 73 in the PFET device.

As it is known in the art, the common property of high-k gatedielectrics is the possession of a larger dielectric constant than thatof the standard oxide (SiO₂) gate insulator material, which has a valueof approximately 3.9. FIG. 1 shows an embodiment when the layers of thefirst and second high-k materials 10, 11 are of a different kind. Inembodiments of the present invention the first and second layers ofhigh-k materials 10, 11 may be ZrO₂, HfO₂, Al₂O₃, HfSiO, HfSiON, and/ortheir admixtures. Equally, the embodiments of the present invention maybe such that the first and second high-k materials 10, 11 are of a same,common material. In a typical embodiment of the invention a commonhigh-k material that may be present in both gate insulators 10, 11 isessentially HfO₂.

Each gate insulator besides the high-k dielectric layers 10, 11 may haveother components, as well. Often in embodiments of the present inventiona very thin, less than about 1 nm, chemically formed oxide layer 12 maybe present between the first and second high-k dielectric layers, 10, 11and the device body 50. However, any and all inner structure, or thelack of any structure beyond simply containing a high-k dielectric, foreither the NFET or the PFET gate insulators is within the scope of theembodiments of the present invention. In exemplary embodiments of thepresent invention, HfO₂ would be used for both the first and the secondhigh-k dielectric layer, 10, 11 and the HfO₂ would be covering a thinchemical SiO₂ layer 12. The EOT of such a gate insulator may be betweenabout 0.4 nm to 1.2 nm.

The NFET gate stack 55 and the PFET gate stack 56 in typical embodimentsof the present invention are multilayered structures. Besides thediscussed common identical portion 110, they would usually includesilicon portions 58, 59 in polycrystalline and possibly also inamorphous forms. The top of the gate stacks usually consist of silicidelayers 42. Any, and all such multilayered structures complementing thecommon identical portion 110 are included in the scope of the presentinvention.

As the consequence of the two type of devices having the commonidentical portion 110, which was patterned from commonly depositedlayers, the circuit structure 100 may be implemented with buttedelectrodes, or junctions. The term “butted junction” is well known inthe electronic arts, it means that two electrodes, such as thesource/drain junctions from adjacent PFET and NFET devices, are disposedside by side in direct physical contact, without isolation regionbetween them. Without the isolation regions the circuit density can behigher than with isolation regions, since less chip area is used up bythe isolation structures.

An alternate term for source and drain junctions are source and drainelectrodes, expressing the electrical connection between the channel andthe source and the drain. Also, in deeply submicron generations of FETs,the source/drain junctions and the body of the classical FETs, namely n⁺regions forming a junction with a p-type device body for NFET, and p⁺region forming a junction with an n-type device body, are undergoing amyriad of changes and may not resemble textbook cases. Embodiments ofthe present invention are not limited by any particular realization ofthe NFET and PFET electrodes. Any an all variations, from all metallicShottky barrier electrodes, to the above exemplified classicaljunctions, to electrodes penetrating down to buried insulating layers,and odd shaped structures belonging to various FIN device bodies, areall within the scope of embodiments of the present invention. The shapeand actual realization of the electrodes is not significant.

FIG. 1 displays, without limiting the general scope, an electrodearrangement often used in FET devices. In the figure, the silicidedregions, shown in dark, penetrate deeper than the doped regions, again atypical arrangement in FETs, and shown without the intent of limitation.For all electrodes, for both sources and drains, and for both NFET andPFET devices, if the doped part of the electrodes are given certainindicator numbers, then the silicided part of the same electrodes aregiven the same numbers primed, for instance, 83 and 83′ for one of thePFET electrodes.

The NFET electrodes 80 and 80′ and 81 and 81′, including a firstelectrode 80, 80′, are adjoining the n-channel 44, and are capable ofbeing in electrical continuity with the n-channel 44. PFET electrodes 82and 82′ and 83 and 83′, including a second electrode 82, 82′, areadjoining the p-channel 46, and are capable of being in electricalcontinuity with the p-channel 46. Electrical current can flow betweenthe electrodes of either of the devices, through the respective channel,when the source to gate voltage exceeds the threshold voltage value. Asshown in the figure, the sides of the electrodes facing away from thechannel are butted. The first electrode 80, 80′ and the second electrode82, 82′ are butted against one another in direct physical contact. Ifdesired, isolation structures, of course, can be introduced betweendevices. The presented fabrication method allows the butting ofelectrodes, it does not necessarily requires it. As illustrated, forinstance the NFET junction 81, 81′ is not butted against anotherjunction, but is confined by an isolation structure 99, shown as anoxide shallow trench scheme, known in the art. More detailedpresentation of butted junctions for short FET devices with high-kdielectrics and metal gates is given in U.S. patent application Ser. No.11/745,994, filed on May 8, 2007, titled: “Devices with Metal Gate,High-k Dielectric, and Butted Electrodes”, incorporated herein byreference.

FIG. 1 further shows the presence of a first dielectric layer 60overlaying the NFET gate stack 55 and the vicinity of the NFET gatestack. The first dielectric layer 60 and the n-channel 44 are in atensile stress, the tensile stress is being imparted by the firstdielectric layer 60 onto the n-channel 44. Similarly, a seconddielectric layer 61 is overlaying the PFET gate stack 56 and thevicinity of the PFET gate stack. The second dielectric layer 61 and thep-channel 46 are in a compressive stress, the compressive stress isbeing imparted by the second dielectric layer 61 onto the p-channel 46.The term vicinity indicates that the gate stacks 55, 56 are fully, orpartially, surrounded by the stressed dielectric layers. The vicinity ofthe stacks 55, 56 may include the source/drain regions 80, 80′, 81, 81′,82, 82′, 83, 83′, and possibly include isolation structures 99, and theSi body material 50.

Inducing stress of a desirable kind in channels of FET devices by theuse of stressed dielectric layers has been known in the art. Theproperties of charge transport in Si based materials is such that FETperformance improves if an n-channel is under tensile stress, and ap-channel is under compressive stress. As discussed above, in typicalembodiments of the present invention this performance enhancing patternis followed.

In exemplary embodiments of the present invention both the first 60 andthe second 61 dielectric layers are essentially nitride (SiN) layers,which can be deposited as either under compressive, or under tensilestress. The thickness of the stressed nitride layers are usually aboutbetween 20 nm and 150 nm.

The absolute values of the saturation thresholds of the NFET and thePFET devices are higher than about 0.4 V, which values assure low powerconsumption. The desired, and roughly symmetric—meaning about equal inabsolute size—threshold values are arrived at after purposefulprocessing.

Following customary terminology, the discussion of the PFET thresholdsare at times referred to without the term “absolute value”, or shownwith an explicit negative sign. It is understood, however that PFETthresholds have negative voltage values. After the forming, and thepatterning of the gate stacks, the thresholds are at about mid-gapvalues, in the range of about 0.5 V-0.7 V. Upon processing the caplayers 72, 73 both thresholds shift in the direction of an n-type Si gapvalue, entailing the lowering of the NFET device threshold, and araising of the PFET device threshold. However, exposing to oxygen onlythe second high-k dielectric 11 of the PFET gate stack, shifts the PFETdevice threshold toward a p-type Si gap value, namely lowering the PFETdevice threshold, while leaving the NFET device threshold unchanged.With the right tuning of the processes one may reach roughly quarter-gapthreshold values for both type of devices. See, for instance: “2005Symposium on VLSI Technology Digest of Technical Papers, Pg. 230, by E.Cartier”, and “V. Narayanan et al in “IEEE VLSI Symposium”, p. 224,(2006)” both publications incorporated herein by reference. In typicalembodiment of the present invention the absolute values of thesaturation thresholds of the NFET and the PFET devices are between about0.40 V and about 0.65 V.

The layer of the first high-k material 10 has a first concentration ofcharges and the layer of the second high-k material 11 has a secondconcentration of charges, which concentrations include possible chargeson the interfaces of the high-k materials. Having lowered the thresholdof the PFET device, the second concentration is more negative, namely isshifted in the negative direction in comparison to the firstconcentration. This shift indicates a history of oxygen exposure of thelayer of the second high-k material 11.

It is understood that FIG. 1, as all figures, is only a schematicrepresentation. As known in the art, there may be many more, or less,elements in the structures than presented in the figures, but thesewould not affect the scope of the embodiments of the present invention.

Further discussions and figures may present only those processing stepswhich are relevant in yielding the structure of FIG. 1. Manufacturing ofNFET, PFET, and CMOS is very well established in the art. It isunderstood that there are a large number of steps involved in suchprocessing, and each step might have practically endless variationsknown to those skilled in the art. It is further understood that thewhole range of known processing techniques are available for fabricatingthe disclosed device structures, and only those process steps will bedetailed that are related to embodiments of the present invention.

FIG. 2 shows a schematic cross section of a stage in the processingwhere various layers, including common layers, have already beendeposited. This figure shows a variation on the embodiment depicted onFIG. 1. Instead of butted junctions, it shows the already discussedembodiment when an isolation structure 99 is between the future place ofthe first electrode 80, 80′ and the second electrode 82, 82′. As alsopresented in representative embodiments earlier, in this figure thefirst high-k material 10 and the second high-k material 11 are patternedfrom a commonly deposited identical high-k material layer 15. The thinchemical oxide layer 12 interfaces between the bodies 50 and theidentical material layer 15. FIG. 2 also shows that the high-k materiallayer 15 is overlaid with a layer of a cap material 76 in a manner thatthe identical high-k material layer 15 is in direct physical contactwith the layer of the cap material 76. If in an alternate embodiment thehigh-k material would not be identical for both type of devices, thelayer of the cap material 76 would similarly overlay differing layers ofa first and second high-k material. The cap material layer 76 isoverlaid with a gate metal layer 75, in a manner that the layer of thecap material 76 and the a gate metal layer 75 are in direct physicalcontact.

The metal 75 layer may be covered with further material layers,typically, but not necessarily of polycrystalline and/or amorphous Si,that will be part of the gate stacks 55, 56 after patterning. Details ofsuch layers are not significant for the embodiments of the presentinvention and they are shown lumped together 57.

During the forming of all these layers: the thin oxide 12, the high-kdielectric 15, the cap 76, the gate metal 75, and the additional ones57, the use of not even a single mask may have been necessary. All theselayers have been blanket formed, or disposed, over the regions of boththe PFET devices and the NFET devices. Finally, FIG. 2 shows that inpreparation for patterning of the gate stacks, masking layers 125 asknow in the art, have been formed, as well.

FIG. 3 shows a schematic cross section of a stage in the processingafter gate patterning. The gate stacks 55, 56 have been created byetching methods know in the art. As shown, the high-k dielectric layerand the thin oxide layer 12 are still in place. As one skilled in theart may know, there are several possibilities for dealing with suchlayers. They may be etched together with the gate stacks, or etchedlater, or possibly left in place. All such eventualities are included inthe scope of the present invention. FIG. 3 shows the result of thepatterning of the cap material layer 76 and of the gate metal layer 75,with the result of having produced the portion 110 which is identical inthe NFET device and in the PFET device. These layers may be covered withfurther material layers, typically, but not necessarily ofpolycrystalline and/or amorphous Si, that will be part of the gatestacks 55, 56 after patterning. Details of such layers are notsignificant for the embodiments of the present invention and they areshown lumped together 57.

FIG. 4 shows a schematic cross section of a state of the processing forembodiments of the present invention where gate stacks and electrodeshave already been formed. Following the patterning of the gate stacks55, 56 the NFET and PFET devices have reached the depicted stage in thefabrication by the use of processing steps known in the art. Spacers 65,66 are shown because they are elements, as know in the art, involved insource/drain fabrication and the silicidation of electrodes, 80′, 81′,82′, 83′ and of the gates 42. The spacers 65, 66 typically are made ofnitride. The threshold of the NFET device has been decreased and thethreshold of the PFET device increased with the help of the cap layers72, 73 in the identical portions 110. With this shift in the thresholds,at this stage in the processing, the NFET device thresholds isessentially set, and it is not intended to change further.

The electrodes of the devices have already been through a high thermalbudget process. In FET processing, typically the largest temperaturebudgets, meaning temperature and time exposure combinations, are reachedduring source/drain electrodes fabrication. Since the sources and drainshave already been fabricated, the structure of FIG. 4 has alreadyreceived such high temperature fabrication steps, and the structure willnot have to be exposed to a further large temperature budget treatment.From the perspective of embodiments of the present invention, exposureto a high temperature budget means a comparable heat treatment as theone used in the source/drain fabrication.

FIG. 5 shows a schematic cross section of a following stage in theprocessing of embodiments of the present invention where spacers havebeen removed. In standard FET fabrication the spacers 65, 66 wouldremain in place through the many following processing steps. Inembodiments of the present invention, however, the final thresholdadjustment by oxygen exposure of the PFET device is yet to be done. Thespacer for the PFET device 66, which is made of nitride, would blockoxygen penetration to the high-k material of the gate dielectric 11.Accordingly, the spacer of the PFET device may have to be removed. Thespacer of the NFET device 65 could stay in place as a barrier againstoxygen penetration. However, in representative embodiments of thepresent invention the NFET device spacer 65 is also removed, to bereplaced with a dielectric layer which is preferably under appropriatestress. In representative embodiments of the present invention the dualroles of protecting the high-k dielectric 10 of the NFET device, and ofproviding stress for higher performance, may be combined into one.Accordingly, usually, but necessarily, both spacers 65, 66 are beingremoved. The removal is done by etching in manners known in the art. Forinstance, glycerated buffered hydrofluoric acid with ratios of 5:1:1.6etches nitride selectively versus silicon, oxide, and metal, whichmaterials may be exposed on the wafer surface while the nitride is beingetched.

FIG. 6 shows a schematic cross section of a stage in the processing ofembodiments of the present invention where an oxygen blocking stresseddielectric layer 60 is overlaying the NFET device, and the circuitstructure is exposed to oxygen 101. After the application of properblocking masks, as known in the art, the NFET device is overlaid by afirst dielectric layer 60 covering the first gate stack 55 and thevicinity of the NFET gate stack. The first dielectric layer 60 and then-channel 44 are in tensile stress, which tensile stress is imparted bythe first dielectric layer 60 onto the n-channel 44. Also, the firstdielectric layer 60 is so selected to be a blocker against oxygenpenetration. In typical embodiments of the present invention the firstdielectric layer 60 is a nitride (SiN) layer. FIG. 6 shows the step ofoxygen exposure 101, as well. This exposure may occur at low temperatureat about 200° C. to 350° C. by furnace or rapid thermal anneal. Theduration of the oxygen exposure 101 may vary broadly from approximately2 minutes to about 150 minutes. For the duration of the exposure oxygenis blocked by the first dielectric layer 60 from penetrating to thelayer of the first high-k material 10 of the NFET device, but oxygen iscapable of penetrating to the layer of the second high-k material 11 ofthe PFET device. In the embodiment shown in FIG.6 the first and secondlayers of the high-k material 10, 11 are of an identical material,typically HfO₂. The amount of threshold shift for the PFET devicedepends on the oxygen exposure parameters, primarily on the temperatureand duration of the procedure. In exemplary embodiments of the presentinvention the size of the threshold shift is so selected that the finalPFET device threshold corresponds also to an approximately quarter-gapvalue workfunction gate. In representative embodiments of the presentinvention the absolute values of the saturation thresholds of the NFETand the PFET devices are between about 0.40 V and about 0.65 V.

After the oxygen exposure step, the PFET device is overlaid with asecond dielectric layer 61, in compressive stress, which is imparted tothe p-channel 46. This second dielectric layer 61 may be laid down in aconformal or a non-conformal manner. In exemplary embodiments of thepresent invention the second dielectric layer 61 is a nitride (SiN)layer. Stressed dielectric layers and their implementation by SiN isdiscussed in more detail in U.S. patent application Ser. No. 11/682,554,filed on Mar. 6, 2007, titled: “Enhanced Transistor Performance byNon-Conformal Stressed Layers”, incorporated herein by reference. Withthe second dielectric layer 61 in place, one arrives to the structuredisplayed and discussed in reference to FIG. 1.

The circuit structure, and its wiring, may be completed with standardsteps known to one skilled in the art.

FIG. 7 shows a symbolic view of a processor containing at least one CMOScircuit according to an embodiment of the present invention. Such aprocessor 900 has at least one chip 901, which contains at least onecircuit structure 100, with at least one NFET and one PFET having high-kgate dielectrics, gate stacks comprising a common identical portionhaving at least a gate metal, a cap layer. The saturation thresholds ofthe FETs are optimized for low power consumption, in absolute valuesbeing above approximately 0.4 V. The processor 900 may be any processorwhich can benefit from embodiments of the present invention, whichyields high performance at low power. Representative embodiments ofprocessors manufactured with embodiments of the disclosed structure aredigital processors, typically found in the central processing complex ofcomputers; mixed digital/analog processors, typically found incommunication equipment; and others.

In the foregoing specification, the invention has been described withreference to specific embodiments. However, one of ordinary skill in theart appreciates that various modifications and changes can be madewithout departing from the scope of the present invention as set forthin the claims below. Accordingly, the specification and figures are tobe regarded in an illustrative rather than a restrictive sense, and allsuch modifications are intended to be included within the scope ofpresent invention.

Benefits, other advantages, and solutions to problems have beendescribed above with regard to specific embodiments. However, thebenefits, advantages, solutions to problems, and any element(s) that maycause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as a critical, required, or essentialfeature, or element, of any or all the claims.

Many modifications and variations of the present invention are possiblein light of the above teachings, and could be apparent for those skilledin the art. The scope of the invention is defined by the appendedclaims.

1. A circuit structure, comprising: at least one NFET device, said NFETdevice comprises an n-channel hosted in a Si based material, and an NFETgate stack overlapping said n-channel; at least one PFET device, saidPFET device comprises a p-channel hosted in said Si based material, anda PFET gate stack overlapping said p-channel; wherein said NFET gatestack and said PFET gate stack each has a portion which is identical insaid NFET device and in said PFET device, wherein said portion comprisesat least a gate metal layer and a cap layer, wherein said gate metallayer and said cap layer are directly interfacing with each other;wherein said NFET device further comprises an NFET gate insulator,wherein said NFET gate insulator comprises a layer of a first high-kmaterial, wherein said layer of said first high-k material directlyinterfaces with said cap layer in said NFET device; wherein said PFETdevice further comprises a PFET gate insulator, wherein said PFET gateinsulator comprises a layer of a second high-k material, wherein saidlayer of said second high-k material directly interfaces with said caplayer in said PFET device; and wherein absolute values of the saturationthresholds of said NFET and said PFET devices are higher than about 0.4V.
 2. The circuit structure of claim 1, further comprising: a firstdielectric layer overlaying said NFET gate stack and a vicinity of saidNFET gate stack, wherein said first dielectric layer and said n-channelare in a tensile stress, wherein said tensile stress is imparted by saidfirst dielectric layer onto said n-channel; and a second dielectriclayer overlaying said PFET gate stack and a vicinity of said PFET gatestack, wherein said second dielectric layer and said p-channel are in acompressive stress, wherein said compressive stress is imparted by saidsecond dielectric layer onto said p-channel.
 3. The circuit structure ofclaim 2, wherein said first dielectric layer and said second dielectriclayer are both composed essentially of SiN.
 4. The circuit structure ofclaim 1, wherein said gate metal layer is selected from the groupconsisting of W, Mo, Mn, Ta, TaN, TiN, WN, Ru, Cr, Ta, Nb, V, Mn, Re,and their admixtures.
 5. The circuit structure of claim 4, wherein saidgate metal layer is composed essentially of TiN, and said cap layercomprises lanthanum (La).
 6. The circuit structure of claim 1, whereinsaid layer of said first high-k material has a first concentration ofcharges and said layer of said second high-k material has a secondconcentration of charges, wherein said second concentration is morenegative than said first concentration, whereby indicating a history ofoxygen exposure of said layer of said second high-k material.
 7. Thecircuit structure of claim 6, wherein said first high-k material andsaid second high-k material are both essentially of HfO₂.
 8. The circuitstructure of claim 1, wherein said absolute values of the saturationthresholds of said NFET and said PFET devices are between about 0.45 Vand about 0.65 V.
 9. The circuit structure of claim 1, wherein said NFETdevice further comprises NFET electrodes, including a first electrode,wherein said NFET electrodes adjoin said n-channel and are capable ofbeing in electrical continuity with said n-channel, and wherein saidPFET device further comprises PFET electrodes, including a secondelectrode, wherein said PFET electrodes adjoin said p-channel and arecapable of being in electrical continuity with said p-channel, andwherein said first electrode and said second electrode are buttedagainst one another in direct physical contact.
 10. The circuitstructure of claim 1, wherein said circuit structure is characterized asbeing a CMOS structure.